Adding a test for exponential decay.

Also fixes a bug revealed by the test. RE:#722

src/natcap/invest/urban_nature_access.py

@@ -2063,7 +2063,8 @@ def _kernel_exponential(distance, max_distance):
     """
     kernel = numpy.zeros(distance.shape, dtype=numpy.float32)
     pixels_in_radius = (distance <= max_distance)
-    kernel[pixels_in_radius] = numpy.exp(-distance / max_distance)
+    kernel[pixels_in_radius] = numpy.exp(
+        -distance[pixels_in_radius] / max_distance)
     return kernel
 
 

tests/test_urban_nature_access.py

@@ -309,6 +309,20 @@ class UNATests(unittest.TestCase):
         numpy.testing.assert_allclose(
             expected_array, kernel)
 
+    def test_exponential_kernel(self):
+        """UNA: Test the exponential decay kernel."""
+        from natcap.invest import urban_nature_access
+
+        max_distance = 3
+        distance = numpy.array([0, 1, 2, 3, 4])
+        kernel = urban_nature_access._kernel_exponential(
+            distance, max_distance)
+        # Regression values are calculated by hand
+        expected_array = numpy.array(
+            [1, 0.71653134, 0.5134171, 0.36787945, 0])
+        numpy.testing.assert_allclose(
+            expected_array, kernel)
+
     def test_greenspace_budgets(self):
         """UNA: Test the per-capita greenspace budgets functions."""
         from natcap.invest import urban_nature_access